The present invention relates to a method for the production of a multilayer of an electrode and an electrolyte membrane suitable for use in fuel cells.
In general, fuel cells include a unit of an electrode and an electrolyte membrane.
It is a conventional practice in producing the electrode-electrolyte units that a hydrogen reduction catalyst carried on carbon is mixed with an electrolyte, the resultant electrode paste is applied on carbon paper and heated to form an electrode, and an electrolyte filmy membrane is sandwiched between two electrodes so that a three-layered anode/electrolyte/cathode unit is obtained by hot pressing the superposed membranes.
However, such three-layer bonding methods as mentioned above have technical problems that, for example, interlaminar adhesion is poor, the three-layer integral bonding takes long to complete and mass production cannot be expected since the layers have to be produced individually.
Further, the electrolyte membrane of high heat resistance, for which a demand has been growing recently, has a problem that its thermoplasticity is so insufficient that some restrictions should apply in thermal adhesion.
With such a problem, it has been proposed that a coating solution obtained by dissolving a compound for forming an electrolyte membrane in a solvent can be applied on a previously prepared electrode and thereafter dried.
However, such coating solutions containing the dissolved electrolyte of high heat resistance, will penetrate into the electrodes upon application thereon so that sufficient power generation properties cannot be obtained.
It is an object of the invention to obtain multilayers capable of satisfactory power generation properties as an electrode structure by forming an electrolyte membrane on an electrode without causing any penetration of electrolyte into the electrode.
The invention provides the following method of producing a multilayer to achieve the above object.
(1) A method for the production of a multilayer comprising an electrode and an electrolyte membrane, the method comprising:
applying a coating solution (I) which comprises a dissolved or dispersed proton-conductive polymer and a solvent (Is) containing water in amounts from 25 to 60 wt % and an organic solvent in amounts from 75 down to 40 wt %, on an electrode;
applying a coating solution (II) which comprises a dissolved or dispersed proton-conductive polymer and a solvent (IIs) containing water in amounts from 0 to less than 25 wt % and an organic solvent in amounts above 75 wt %, on the first coating without any drying of the first coating; and
drying the coatings to form an electrolyte membrane.
(2) The method for the production of an multilayer according to above (1), wherein the proton-conductive polymer is sulfonated polyarylene.
(3) The method for the production of a multilayer according to above (1) or (2), wherein the solvent (Is) contains water in amounts from 30 to 50 wt % and an organic solvent in amounts from 70 down to 50 wt %.
(4) The method for the production of a multilayer according to any of above (1) to (3), wherein the solvent (IIs) contains water in amounts from 10 to 20 wt % and an organic solvent in amounts from 90 down to 80 wt %.
(5) The method for the production of a multilayer according to any of above (1) to (4), wherein the coating solution (I) contains the proton-conductive polymer in amounts from 0.1 to 10 wt %.
(6) The method for the production of a multilayer according to any of above (1) to (5), wherein the coating solution (II) contains the proton-conductive polymer in amounts from 5 to 20 wt %.
(7) The method for the production of a multilayer according to any of above (1) to (6), wherein the organic solvent is at least one solvent selected from the group consisting of tetrahydrofuran, cyclohexanone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, xcex3-butyrolactone and xcex3-butyrolactam, preferably from the group consisting of tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone.
The methods for the production of multilayers of the invention will be described in detail hereinafter.
In the methods, two kinds of coating solutions are used. They are obtained by dissolving or dispersing the proton-conductive polymer in the solvent which is different from the other in the water content. The coating solution with larger water content is first applied on an electrode, the one with smaller water content is thereafter applied on the undried first coating, and these coatings are dried to form an electrolyte membrane, thus a multilayer comprising the electrode and the electrolyte membrane is obtained.
The electrode for use in the invention is obtained by coating a gas diffusion electrode substrate with a paste which comprises fine particles of catalyst capable of hydrogen reduction supported on conductive porous particles and a proton-conductive polyelectrolyte component.
The conductive porous particles used herein have high structures and large surface areas, e.g., Ketjen black and acetylene black. Examples of the catalyst capable of hydrogen reduction include noble metals, such as platinum, palladium, ruthenium and rhodium; and alloys of these metals and other metals such as chromium, molybdenum, tungsten, titanium, zirconium and cobalt. The amount of the catalyst supported is usually in the range of 10 to 60 wt % based on the conductive porous particles. The paste is applied to a porous gas diffusion electrode substrate, such as carbon paper or carbon cloth, by means of a doctor blade or a spray, so that the electrode is obtained.
The electrode generally ranges from 5 to 100 xcexcm, preferably 5 to 50 xcexcm in the thickness.
Examples of the proton-conductive polymer for the electrolyte membrane include sulfonated polyarylene, sulfonated polyarylene ether, sulfonated polyarylene ketone, sulfonated polybenzimidazole and tetrafluoroethylene copolymers. Of these, sulfonated polyarylene is preferred in order to obtain an electrode structure having good electric characteristics.
The sulfonated polyarylene is prepared by sulfonating a polymer which results from the reaction of a monomer (A) of the following formula (A) with at least one monomer (B) selected from the following monomers (B-1) to (B-4). 
In the formula (A), R and Rxe2x80x2, which may be the same or different, are independently a halogen atom other than a fluorine atom or a xe2x80x94OSO2Z group (Z is an alkyl group, a fluorine-substituted alkyl group or an aryl group).
Examples of the alkyl group indicated by Z include methyl and ethyl; those of the fluorine-substituted alkyl group include trifluoromethyl; and those of the aryl group include phenyl and p-tolyl.
R1 to R8, which may be the same or different, are independently at, least one atom or group selected from the group consisting of hydrogen, a fluorine atom, and alkyl, fluorine-substituted alkyl, allyl and aryl groups.
Examples of the alkyl groups include methyl, ethyl, propyl, butyl, amyl and hexyl. Of these, methyl, ethyl and the like are preferred.
Examples of the fluorine-substituted alkyl groups include trifluoromethyl, perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl and perfluorohexyl. Of these, trifluoromethyl, pentafluoroethyl and the like are preferred.
Examples of, the allyl groups include propenyl.
Examples of the aryl groups include phenyl and pentafluorophenyl.
X is a divalent electron attracting group, e.g., xe2x80x94COxe2x80x94, xe2x80x94CONHxe2x80x94, xe2x80x94(CF2)pxe2x80x94 (p is an integer of 1 to 10), xe2x80x94C(CF3)2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94SOxe2x80x94 and xe2x80x94SO2xe2x80x94.
The electron attracting group can be defined as having a Hammett substituent constant of not less than 0.06 in the case of a phenyl group at the m-position and not less than 0.01 in the case of the p-position.
Y is a divalent electron donating group, e.g., xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 and groups represented by the following formulae: 
n is 0 or a positive integer within the upper limit of 100, preferably within the upper limit of 80.
Examples of the monomer of the formula (A) include 4,4xe2x80x2-dichlorobenzophenone, 4,4xe2x80x2-dichlorobenzanilide, bis(chlorophenyl)difluoromethane, 2,2-bis(4-chlorophenyl)hexafluoropropane, 4-chlorobenzoic acid-4-chlorophenyl, bis(4-chlorophenyl)sulfoxide, bis(4-chlorophenyl)sulfone, corresponding compounds to the above compounds except the chlorine atom is replaced with a bromine atom or an iodine atom, and corresponding compounds to the above compounds except the halogen atom substituted at the 4-position is substituted at the 3-position.
Examples of the monomer of the formula (A) further include 4,4xe2x80x2-bis(4-chlorobenzoyl)diphenyl ether, 4,4xe2x80x2-bis(4-chlorobenzoylamino)diphenyl ether, 4,4xe2x80x2-bis(4-chlorophenylsulfonyl)diphenyl ether, 4,4xe2x80x2-bis(4-chlorophenyl)diphenyl ether dicarboxylate, 4,4xe2x80x2-bis[(4-chlorophenyl)-1,1,1,3,3,3-hexafluoropropyl]diphenyl ether, 4,4xe2x80x2-bis[(4-chlorophenyl)tetrafluoroethyl]diphenyl ether, corresponding compounds to the above compounds except the chlorine atom is replaced with a bromine atom or an iodine atom, corresponding compounds to the above compounds except the halogen atom substituted at the 4-position is substituted at the 3-position, and corresponding compounds to the above compounds except at least one of the groups substituted at the 4-position of diphenyl ether is substituted at the 3-position.
Also available as the monomers of the formula (A) are 2,2-bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]-1,1,1,3,3,3-hexafluoropropane, bis[4-{4-(4-chlorobenzoyl)phenoxy}phenyl]sulfone, and compounds represented by the following formulae: 
For example, the monomers (A) can be synthesized by the following process.
First, bisphenols connected with the electron attracting group are converted into corresponding alkali metal salts of bisphenol by reaction with an alkali metal, such as lithium, sodium or potassium, or an alkali metal compound, such as an alkali metal hydride, an alkali metal hydroxide or an alkali metal carbonate, in a polar solvent which has a high dielectric constant, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, sulfolane, diphenyl sulfone or dimethyl sulfoxide. The alkali metal, etc. is usually used in slight excess based on the hydroxyl groups of the bisphenol, for example usually 1.1 to 2 times equivalent amount, preferably 1.2 to 1.5 times equivalent amount.
Thereafter, the bisphenol is reacted with a halogen-substituted, e.g., fluorine- or chlorine-substituted, aromatic dihalide compound activated by the electron attracting group, such as 4,4xe2x80x2-difluorobenzophenone, 4,4xe2x80x2-dichlorobenzophenone, 4,4xe2x80x2-chlorofluorobenzophenone, bis(4-chlorophenyl)sulfone, bis(4-fluorophenyl)sulfone, 4-fluorophenyl-4xe2x80x2-chlorophenylsulfone, bis(3-nitro-4-chlorophenyl)sulfone, 2,6-dichlorobenzonitrile, 2,6-difluorobenzonitrile, hexafluorobenzene, decafluorobiphenyl, 2,5-difluorobenzophenone or 1,3-bis(4-chlorobenzoyl)benzene, in the presence of a solvent azeotropic with water, such as benzene, toluene, xylene, hexane, cyclohexane, octane, chlorobenzene, dioxane, tetrahydrofuran, anisole or phenetole. From the viewpoint of reactivity, the aromatic dihalide compound is preferably a fluorine compound. But taking the subsequent aromatic coupling reaction into account, the aromatic nucleophilic substitution reaction should be carried out such that both ends of the molecule are terminated with a chlorine atom. The active aromatic dihalide compound is used in amounts 2 to 4 molar times, preferably 2.2 to 2.8 molar times the amount of the bisphenol.
The reaction temperature is in the range of 60 to 300xc2x0 C., preferably 80 to 250xc2x0 C. The reaction time is in the range of 15 minutes to 100 hours, preferably 1 to 24 hours.
Optimally, the active aromatic dihalide is a chlorofluoro compound as shown below that has two halogen atoms different in the reactivity. The use of this compound is advantageous since the fluorine atom will preferentially undergoes the nucleophilic substitution reaction with phenoxide so that the objective chlorine-terminated active compound can be obtained. 
wherein X and Y are as defined in the formula (A).
Other exemplary methods for the synthesis of the monomer (A) include JP-A-2(1990)/159, in which the nucleophilic substitution reaction is jointly carried out with an electrophilic substitution reaction to synthesize the objective flexible compound comprising the electron attracting and electron donating groups.
Specifically, the aromatic bis-halide, such as bis(4-chlorophenyl)sulfone, activated by the electron attracting group is subjected to the nucleophilic substitution reaction with phenol, and the resulting bis-phenoxy substituted compound is subjected to Friedel-Crafts reaction with, e.g., 4-chlorobenzoyl chloride to give the objective compound.
Any of the above-exemplified compounds can be applied to the aromatic bis-halide activated by the electron attracting group. The phenol compound may be substituted, but is preferably unsubstituted from the viewpoints of heat resistance and flexibility. When substituted, the substituted phenol compound is preferably an alkali metal salt. The phenol compound can be substituted with any of the alkali metal compounds listed above. The alkali metal compound is used in amounts 1.2 to 2 molar times the amount of the phenol. In the reaction, the aforesaid polar solvent or the azeotropic solvent with water is employable.
To obtain the objective compound, the bis-phenoxy compound is reacted with an acylating agent chlorobenzoyl chloride in the presence of an activator for the Friedel-Crafts reaction, e.g., Lewis acid such as aluminum chloride, boron trifluoride or zinc chloride. The chlorobenzoyl chloride is used in amounts 2 to 4 molar times, preferably 2.2 to 3 molar times the amount of the bis-phenoxy compound. The Friedel-Crafts reaction activator is used in 1.1 to 2 times equivalent amount based on 1 mol of the active halide compound, such as an acylating agent chlorobenzoic acid. The reaction time is in the range of 15 minutes to 10 hours, and the reaction temperature is in the range of xe2x88x9220 to 80xc2x0 C. As a solvent, chlorobenzene, nitrobenzene or the like that is inactive in the Friedel-crafts reaction can be used.
Examples of such monomers (A) include compounds represented by the following formulae: 
In the above formulae, n is not less than 2, preferably from 2 to 100.
Next, the monomers represented by the formulae (B-1) to (B-4) are described. 
In the above formula, R and Rxe2x80x2, which may be the same or different, denote the same groups as in the formula (A).
R9 to R15, which may be the same or different, are independently at least one atom or group selected from hydrogen, a fluorine atom and an alkyl group.
Examples of the alkyl groups indicated by R9 to R15 include the same alkyl groups as indicated by R1 to R8 in the formula (A).
m is 0, 1 or 2.
X is a divalent electron attracting group selected from the same groups for X in the formula (A).
Y is a divalent electron donating group selected from the same groups for Y in the formula (A).
W is at least one group selected from the group consisting of a phenyl group, a naphthyl group and groups represented by the following formulae (C-1) to (C-3): 
In the above formulae, A denotes an electron donating group or a single bond.
The electron donating group is a divalent electron donating group selected from the same groups for Y in the formula (A).
R16 and R17 are independently an atom or a group selected from the group consisting of hydrogen, an alkyl group and an aryl group.
Examples of the alkyl and aryl groups indicated by R16 and R17 include the same alkyl and aryl groups as indicated by R1 to R8 in the formula (A).
R18 to R26, which may be the same or different, are independently at least one atom or group selected from hydrogen, a fluorine atom and an alkyl group.
q is 0 or 1.
Examples of the monomers of the formula (B-1) include compounds represented by the following formulae: 
More specifically, the compounds of the formula (B-1) can be exemplified with the following compounds: 
Further, corresponding compounds to the above compounds except the chlorine atom is replaced with a bromine atom or an iodine atom are also available. 
In the formulae (B-2) to (B-4), R and Rxe2x80x2 may be the same or different and denote the same groups as in the formula (A)
R27 to R34, which may be the same or different, are independently hydrogen, a fluorine atom, an alkyl group, a fluorine-substituted alkyl group, an aryl group or a group represented by the following formula (D): 
wherein R35 to R43, which may be the same or different, are independently hydrogen, a halogen atom, an alkyl group or a fluorine-substituted alkyl group.
Examples of the alkyl and fluorine-substituted alkyl groups indicated by R27 to R34 and R35 to R43 include the same alkyl and fluorine-substituted alkyl groups as indicated by R1 to R8. Examples of the aryl groups indicated by R27 to R34 include the same aryl groups as indicated by R1 to R8.
X is a divalent electron attracting group selected from the same groups for X in the formula (A).
Y is a divalent electron donating group selected from the same groups for Y in the formula (A).
Examples of the monomers represented by the formula (B-2) include p-dichlorobenzene, p-dimethylsulfonyloxybenzene, 2,5-dichlorotoluene, 2,5-dimethylsulfonyloxybenzene, 2,5-dichloro-p-xylene, 2,5-dichlorobenzotrifluoride, 4-dichloro-2,3,5,6-tetrafluorobenzene, and corresponding compounds to the above compounds except the chlorine atom is placed with a bromine atom or an iodine atom.
Examples of the monomers represented by the formula (B-3) include 4,4xe2x80x2-dimethylsulfonyloxybiphenyl, 4,4xe2x80x2-dimethylsulfonyloxy-3,3xe2x80x2-dipropenylbiphenyl, 4,4xe2x80x2-dibromobiphenyl, 4,4xe2x80x2-diiodobiphenyl, 4,4xe2x80x2-dimethylsulfonyloxy-3,3xe2x80x2-dimethylbiphenyl, 4,4xe2x80x2-dimethylsulfonyloxy-3,3xe2x80x2-difluorobiphenyl, 4,4xe2x80x2-dimethylsulfonyloxy-3,3xe2x80x2,5,5xe2x80x2-tetrafluorobiphenyl, 4,4xe2x80x2-dibromooctafluorobiphenyl and 4,4xe2x80x2-dimethylsulfonyloxyoctafluorobiphenyl.
Examples of the monomers represented by the formula (B-4) include m-dichlorobenzene, m-dimethylsulfonyloxybenzene, 2,4-dichlorotoluene, 3,5-dichlorotoluene, 2,6-dichlorotoluene, 3,5-dimethylsulfonyloxytoluene, 2,6-dimethylsulfonyloxytoluene, 2,4-dichlorobenzotrifluoride, 3,5-dichlorobenzotrifluoride, 1,3-dibromo-2,4,5,6-tetrafluorobenzene, and corresponding compounds to the above compounds except the chlorine atom is replaced with a bromine atom or an iodine atom.
The monomers mentioned above are reacted in the presence of a catalyst to synthesize the polyarylene. The catalyst used herein is a catalyst system containing a transition metal compound. This catalyst system essentially contains (1) a transition metal salt and a compound which becomes a ligand (referred to as the xe2x80x9cligand componentxe2x80x9d hereinafter), or a transition metal complex (including a copper salt) in which a ligand is coordinated, and (2) a reducing agent. A xe2x80x9csaltxe2x80x9d may be added to increase the polymerization rate.
Examples of the transition metal salt include nickel compounds, such as nickel chloride, nickel bromide, nickel iodide and nickel acetylacetonate; palladium compounds, such as palladium chloride, palladium bromide and palladium iodide; iron compounds, such as iron chloride, iron bromide and iron iodide; and cobalt compounds, such as cobalt chloride, cobalt bromide and cobalt iodide. Of these, nickel chloride, nickel bromide and the like are particularly preferred.
Examples of the ligand component include triphenylphosphine, 2,2xe2x80x2-bipyridine, 1,5-cyclooctadiene and 1,3-bis(diphenylphosphino)propane. Of these, triphenylphosphine and 2,2xe2x80x2-bipyridine are preferred. The ligand components can be used singly or in combination of two or more kinds.
Examples of the transition metal complex with a coordinated ligand include
nickel chloride-bis(triphenylphosphine),
nickel bromide-bis(triphenylphosphine),
nickel iodide-bis(triphenylphosphine),
nickel nitrate-bis(triphenylphosphine),
nickel chloride(2,2xe2x80x2-bipyridine),
nickel bromide(2,2xe2x80x2-bipyridine),
nickel iodide(2,2xe2x80x2-bipyridine),
nickel nitrate(2,2xe2x80x2-bipyridine),
bis(1,5-cyclooctadiene)nickel,
tetrakis(triphenylphosphine)nickel,
tetrakis(triphenylphosphite)nickel and
tetrakis(triphenylphosphine)palladium. Of these, nickel chloride-bis(triphenylphosphine) and nickel chloride(2,2xe2x80x2-bipyridine) are preferred.
Examples of the reducing agent employable in the catalyst system include iron, zinc, manganese, aluminum, magnesium, sodium and calcium. Of these, zinc, magnesium and manganese are preferable. These reducing agents can be used in a more activated form by contact with an acid, e.g., an organic acid.
Examples of the xe2x80x9csaltxe2x80x9d employable in the catalyst system include sodium compounds, such as sodium fluoride, sodium chloride, sodium bromide, sodium iodide and sodium sulfate; potassium compounds, such as potassium fluoride, potassium chloride, potassium bromide, potassium iodide and potassium sulfate; and ammonium compounds, such as tetraethylammonium fluoride, tetraethylammonium chloride, tetraethylammonium bromide, tetraethylammonium iodide and tetraethylammonium sulfate. Of these, sodium bromide, sodium iodide, potassium bromide, tetraethylammonium bromide and tetraethylammonium iodide are preferred.
With respect to the proportion of the components, the transition metal salt or the transition metal complex is used in amounts within the range of usually 0.0001 to 10 mol, preferably 0.01 to 0.5 mol, based on 1 mol of the total monomers. If the amount thereof is less than 0.0001 mol, the polymerization reaction does not proceed sufficiently in some cases. Contrary, the amount thereof exceeding 10 mol may result in a lowered molecular weight of the polyarylene.
When the catalyst system contains the transition metal salt and the ligand component, the ligand component is used in amounts within the range of usually 0.1 to 100 mol, preferably 1 to 10 mol, based on 1 mol of the transition metal salt. If the amount thereof is less than 0.1 mol, the catalytic activity sometimes becomes insufficient. Contrary, the amount thereof exceeding 100 mol may result in a lowered molecular weight of the polyarylene.
The amount of the reducing agent is in the range of usually 0.1 to 100 mol, preferably 1 to 10 mol, based on 1 mol of the total monomers. If the reducing agent is used within amounts less than 0.1 mol, the polymerization does not proceed sufficiently in some cases. Contrary, the amount thereof exceeding 100 mol may cause difficulties in purifying the resulting polyarylene.
When the xe2x80x9csaltxe2x80x9d is used, the amount thereof is in the range of usually 0.001 to 100 mol, preferably 0.01 to 1 mol, based on 1 mol of the total monomers. If the salt is used within amounts less than 0.001 mol, the effect of increasing the polymerization rate often cannot be achieved sufficiently. Contrary, the amount thereof exceeding 100 mol may result in difficult purification of the resulting polyarylene.
Exemplary solvents usable in the polymerization include tetrahydrofuran, cyclohexanone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, xcex3-butyrolactone and xcex3-butyrolactam. Of these, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone are preferred. These polymerization solvents are preferably used after dried sufficiently.
The concentration of the total monomers in the polymerization solvent is in the range of usually 1 to 90 wt %, preferably 5 to 40 wt %.
The polymerization temperature is in the range of usually 0 to 200xc2x0 C., preferably 50 to 120xc2x0 C., and the polymerization time is in the range of usually 0.5 to 100 hours, preferably 1 to 40 hours.
By the polymerization of the monomer (A) of the formula (A) and at least one monomer (B) selected from the monomers of the formulae (B-1) to (B-4) as described above, a polymerization solution containing polyarylene is obtained.
The sulfonated copolymer for use in the electrolyte membrane can be obtained by introducing a sulfonic acid group into the above copolymer which has no sulfonic acid group by a conventional technique using a sulfonating agent.
For introduction of a sulfonic acid group, the copolymer having no sulfonic acid group can be sulfonated by use of a conventional sulfonating agent, such as anhydrous sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, sulfuric acid or sodium hydrogen sulfite, under known conditions (see Polymer Preprints, Japan, vol. 42, No. 3, p. 730 (1993), Polymer Preprints, Japan, vol. 42, No. 3, p. 736 (1994), Polymer Preprints, Japan, vol. 42, No. 7, pp. 2490-2492 (1993)).
Namely, the conditions in the sulfonation are such that the copolymer having no sulfonic acid group is reacted with the sulfonating agent in the presence or absence of a solvent. Examples of the solvent include hydrocarbon solvents, such as n-hexane; ether solvents, such as tetrahydrofuran and dioxane; non-protonic polar solvents, such as dimethylacetamide, dimethylformamide and dimethyl sulfoxide; and halogenated hydrocarbons, such as tetrachloroethane, dichloroethane, chloroform and methylene chloride. Although the reaction temperature is not specifically limited, it is in the range of usually xe2x88x9250 to 200xc2x0 C., preferably xe2x88x9210 to 100xc2x0 C. The reaction time is in the range of usually 0.5 to 1000 hours, preferably 1 to 200 hours.
The sulfonated copolymer thus obtained will contain the sulfonic acid group in amounts within the range of 0.5 to 3 mg equivalent/g, preferably 0.8 to 2.8 mg equivalent/g. If the sulfonic acid group is contained in amounts less than 0.5 mg equivalent/g, the proton conductivity cannot be increased. Contrary, when the amount thereof exceeds 3 mg equivalent/g, hydrophilicity is so increased that the resulting polymer becomes water soluble or, if not water soluble, less durable.
The precursor polymer of the above-obtained sulfonated copolymer (i.e., prior to the sulfonation) has a weight-average molecular weight of 10,000 to 1,000,000, preferably 20,000 to 800,000, in terms of polystyrene. If the molecular weight is less than 10,000, the film performance is so insufficient that cracks occur in the dry film, and besides the film suffers poor strength. If the molecular weight exceeds 1,000,000, the polymer will have insufficient solubility and will increase viscosity of the solution, resulting in bad processability.
In the method for producing multilayers according to the invention,
a coating solution (I) which comprises the proton-conductive polymer in a dissolved or dispersed state and a solvent (Is) containing water in amounts from 25 to 60 wt % and an organic solvent in amounts from 75 down to 40 wt %, is applied on the electrode to form a first coating;
a coating solution (II) which comprises the proton-conductive polymer in a dissolved or dispersed state and a solvent (IIs) containing water in amounts from 0 to less than 25 wt % and an organic solvent in amounts above 75 wt %, is applied on the wet first coating without any drying of the first coating to form a second coating; and
the coatings are dried to form an electrolyte membrane, so that a multilayer comprising the electrode and the electrolyte membrane is produced.
The proton-conductive polymers dissolved or dispersed individually in the coating solution (I) or (II) may be the same or different, and the organic solvents contained individually in the solvent (Is) or (IIs) also may be the same or different.
The coating solution (I) for direct application on the electrode ideally contains the proton-conductive polymer in amounts from 0.1 to 10 wt %, preferably 0.5 to 2 wt %, provided that the components for the coating solution (I) amount to 100 wt %.
The solvent (Is) ideally contains water in amounts from to 60 wt %, preferably 30 to 50 wt %, and the organic solvent in amounts from 75 down to 40 wt %, preferably from 70 down to 50 wt %, provided that water and the organic solvent amount to 100 wt %.
Examples of the organic solvent include tetrahydrofuran, cyclohexanone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, xcex3-butyrolactone and xcex3-butyrolactam. Of these, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone are preferred. These organic solvents can be used singly or in combination of two or more kinds.
When the water content in the solvent (Is) for the coating solution (I) is less than 25 wt %, effects of suppressed penetration of the subsequently-applied coating solution (II), particularly the electrolyte membrane component, into the electrode will be deteriorated. On the other hand, the water content exceeding 60 wt % will cause ununiform dissolution or dispersion of the sulfonated polyarylene so that the homogenous coating can be hardly obtained.
The coating solution (I) can be applied on the electrode by bar coating or spray coating, preferably spray coating.
The wet coating of the coating solution (I) ranges from usually 2 to 100 xcexcm, preferably 10 to 30 xcexcm in the thickness.
After the coating solution (I) is applied on the electrode, the coating solution (II) is applied on the wet coating without any drying.
The coating solution (II) ideally contains the proton-conductive polymer in amounts from 5 to 20 wt %.
The solvent (IIs) ideally contains water in amounts from 0 to less than 25 wt %, preferably 10 to 20 wt %, and the organic solvent in amounts above 75 wt %, preferably from 80 to 90 wt %, provided that water and the organic solvent amount to 100 wt %.
Examples of the organic solvent used herein are as listed for the coating solution (I).
When the water content in the solvent (IIs) for the coating solution (II) is more than 25 wt %, the sulfonated polyarylene cannot have sufficiently high concentration in the coating solution so that the resulting electrolyte membrane will be unsatisfactory.
The coating solution (II) is preferably applied by means of a coater. Once applied, the coatings are dried at usually 50 to 180xc2x0 C., preferably 100 to 150xc2x0 C.
The dry film obtained from the coating solution (II) ranges from 10 to 100 xcexcm, preferably 30 to 50 xcexcm in the thickness.
The dry electrolyte membrane obtained from the coating solutions (I) and (II) ideally ranges from 10 to 200 xcexcm, preferably 20 to 80 xcexcm in the thickness.
As mentioned above, the coating solution (I) which contains the solvent (Is) of the water content within 25 to 60 wt % is applied first, and subsequently the coating solution (II) which contains the solvent (IIs) of the water content within 0 to less than 25 wt % is applied on the undried first coating. As a result, the coating of the coating solution (II) is formed on the coating of the coating solution (I). Therefore the electrolyte membrane component in the coating solution (II) solidifies (phase transition) on the coating of the coating solution (I), so that the electrolyte membrane component in the coating solution (II) can be prevented from penetrating into the electrode.
In the invention, a third coating solution (III) may be further applied on the coating of the coating solution (II).
The coating solution (III) is obtained by dissolving the sulfonated polymer in a mixed solvent consisting of an alcohol with a boiling point of 100xc2x0 C. or below and an organic solvent with a boiling point of above 100xc2x0 C.
Examples of the sulfonated polymer used herein include the same sulfonated polymers used in production of the coating solutions.
Exemplary alcohols with boiling points of 100xc2x0 C. or below include methanol, ethanol, propanol and isopropyl alcohol.
Exemplary organic solvents with boiling points of above 100xc2x0 C. include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, xcex3-butyrolactone, tetramethylurea, dimethyl sulfoxide, hexamethylphosphoric triamide and sulfolane.
The weight ratio between the alcohol with a boiling point of 100xc2x0 C. or below and the organic solvent with a boiling point of above 100xc2x0 C., is in the range of 5-75:95-25, with the proviso that the total is 100.
The concentration of the sulfonated polymer in the coating solution (III) is ideally in the range of 1 to 50 wt %, preferably 3 to 30 wt %.
The coating solution (III) can be applied on the coatings over the electrode by, for example, bar coating or spray coating. The thickness of the coating of the coating solution (III) is in the range of 1 to 100 xcexcm.
After the application of the coating solution (III), the coatings are dried by heating at temperatures from 50 to 200xc2x0 C., preferably 50 to 150xc2x0 C., for a period of 15 minutes to 3 hours, preferably 30 minutes to 2 hours.